It has been proposed previously that a common cofactor exists in most, if not all, molybdoenzymes. We plan to study the synthesis, structure, and function of this cofactor using nitrogenase as a model system. Mutant strain of Klebsiella pneumoniae and Azotobacter vinelandii defective in production of the cofactor will be used to assay the cofactor during purification from homogeneous preparations of component I of nitrogenase. We will study the interaction between the cofactor an inactive component I from cells grown with tungstate instead of molybdate. Genes involved with production of the cofactor will be analyzed in K. pheumoniae and regulation of cofactor synthesis also will be examined. Comparison between the cofactor from other sources such as xanthine oxidase and nitrate reductase will be undertaken.